The invention relates to removal of stray debris from fluidized sand beds. The beds are used in fabrication of molds used in metal casting. The debris, if not removed, can damage the patterns from which the molds are being made.
In the metal casting art, metal is poured into a mold. The mold is generally constructed of a high melting-point ceramic material. In the process of making the mold, a wax replica, or pattern, of the actual item to be cast is first made. Object 3 in FIG. 1 represents the pattern. The pattern 3 is dipped into a liquid ceramic slurry 6, contained in tank 9. Conceptually, the slurry 6 can be viewed as a thin liquid plaster.
Next, the pattern is removed from the slurry 6, and, while still wet with a coating of the slurry, inserted into a fluidized bed 12 of sand, in tank 14. The sand is fluidized by jets of compressed air (not shown) or other gas, which agitate the sand and cause the sand particles to become suspended in the tank 14.
A problem arises at this point, because debris tends to get deposited into the fluidized bed. This debris is set into motion by the fluidized sand 12, and the moving debris can collide with the pattern 3 and knock off parts of the pattern 3, thereby creating even more debris. For example, as shown in FIG. 2, a section 15 of the pattern 3 is shown as being broken off, and now contained in the fluidized bed 12. Prior to breakage, section 15 formed phantom part 16 of the pattern 3.
In the prior art, the debris was typically removed by persons who, in essence, sifted the debris out of the sand, using sieves or screens. However, this process was unable to remove all debris. One reason is that the tanks 14 are deep and wide, compared with the size of the sieves and screens used. Also, the presence of the fluidized sand reduces visibility, so that the debris-removal operation involves a somewhat random sifting process of various areas of the tanks.
In addition, if larger sieves or screens were to be used to mitigate the problem just stated, the larger sieves and screens represented larger weights which the persons must manipulate and lift. In industry, requiring personnel to lift large weights is not favored, because of possible injury to the persons performing the lifting.
The Inventors have developed a system for more effectively cleaning the tank 14 which contains the fluidized bed of sand.
In one form of the invention, a helical screen is dipped into the fluidized bed, and then rotated. Fluidized sand flows through the screen, but debris does not, and is captured.